1. Field of the Invention
The present invention relates to liquid crystal displays (LCDs). More specifically, the present invention relates to methods and apparatus for forming organic alignment layers on a substrate for aligning liquid crystal molecules in multi-domain vertical alignment liquid crystal displays.
2. Discussion of Related Art
Liquid crystal displays (LCDs), which were first used for simple monochrome displays, such as calculators and digital watches, have become the dominant display technology. LCDs are used routinely in place of cathode ray tubes (CRTs) for both computer displays and television displays. Various drawbacks of LCDs have been overcome to improve the quality of LCDs. For example, active matrix displays (using thin-film transistors) replaced passive matrix displays to improve resolution, contrast ratio, viewing angle, response time and reduce ghosting.
However, the primary drawbacks of conventional LCDs are the viewing angle is very narrow and the contrast ratio is low. Even the viewing angle of active matrixes is much smaller than the viewing angle of a conventional cathode ray tube (CRT) display. Specifically, a viewer directly in front of an LCD receives a high quality image; however viewers to the side of the LCD would not receive a high quality image. Multi-domain vertical alignment liquid crystal displays (MVALCDs) were developed to improve the viewing angle and contrast ratio of LCDs. However, the primary drawback of MVA LCDs is the high cost of manufacturing LCDs. FIGS. 1(a)-1(b) illustrate the basic structure and functionality of a pixel of a multi Multi-domain vertical alignment liquid crystal displays (MVALCDs) 100. For clarity, MVALCD 100 of FIGS. 1(a)-1(b) is described for grayscale operation.
MVALCD 100 has a first polarizer 105, a first substrate 110, a first electrode 120, a first alignment layer 125, liquid crystals 135, liquid crystals 137, a second alignment layer 140, a second electrode 145, a second substrate 150, a second polarizer 155, and protrusions 160. Alignment layers 125 and 140 are typically formed using a polyimide (PI) film coating. A light source (not shown) sends light from beneath first polarizer 105, which is attached to first substrate 110. The polarization of the first polarizer 105 is generally directed in a first direction and the polarization of the second polarizer 155 is directed perpendicularly to first polarizer 105. Thus, light from the light source would not pass through both first polarizer 105 and second polarizer 155 unless the polarization of the light were to be rotated by 90 degrees between first polarizer 105 and second polarizer 155. For clarity, very few liquid crystals are shown. In actual displays, liquid crystals are rod like molecules, which are approximately 5 angstroms in diameter and 20-25 angstroms in length. Thus, there are over 10 million liquid crystal molecules in a pixel that is 100 μm width by 300 μm length by 3 μm height.
In FIG. 1(a), liquid crystals 135 and 137 are vertically aligned. Specifically, alignment layers 125 and 140 align the liquid crystals in the desired resting position, which is a vertical alignment. In the vertical alignment, liquid crystals 135 and 137 would not rotate the polarization of the light from the light source. Therefore, second polarizer 155 blocks the light that was polarized in a first direction by first polarizer 105. Thus, pixels in MVALCD 100 can provide a completely optical black state. Consequently MVALCD 100 can provide a very high contrast ratio for all color and all cell gaps. As illustrated in FIG. 1(b), when an electric voltage is applied between first electrode 120 and second electrode 145, liquid crystals 135 and 137 reorient to a tilted position. Specifically, liquid crystals 135 tilt to the left to form a first domain while liquid crystals 137 tilt to the right to form a second domain due to protrusions 160. Liquid crystals in the tilted position rotate the polarization of the polarized light coming through first polarizer 105 by ninety degrees so that the light can then pass through second polarizer 155. The amount of tilting, which controls the rotation of the polarization of the light and thus the amount of light passing through the LCD (i.e., brightness of the pixel), is proportional to the applied voltage of the electric field. Having multiple domains (i.e. liquid crystals 135 and liquid crystals 137) increases the viewing angle of the MVALCD. Generally, a single thin-film-transistor (TFT) is used for each pixel. However for color displays, a color pixel is divided into 3 color components and a separate TFT is used for each color component (typically, Red, Green, and Blue)
The primary drawback of MVA LCDs is the high cost of manufacturing LCDs. While the material cost of polyimide is very low, the process to form of alignment layers 125 and 140 is very costly. Furthermore, conventionally polyimide processes are prone to dust and particle contamination and thus require expensive cleaning and process equipment. Reducing the fabrication cost of the alignment layer can greatly reduce the overall cost of manufacturing liquid crystal displays. In addition, conventional MVA alignment layer fabrication processes process a single wafer at a time, which results in a very low throughput.
Another common method to create alignment layers for MVA LCDs is the thermal evaporation of an inorganic material, such as SiO2 (Silicon Dioxide) or SiO and SiOx (Silicon Monoxide), on the substrates in a high vacuum chamber. This process can produce a stable vertical LC alignment; however, the process uses a high vacuum chamber and is applicable to only small substrates (typically smaller than 10-in in size). The resulting LC alignment is sensitive to the surface cleanliness and topology, and strongly depends on the SiO2 evaporation angle. Only certain evaporation angles can produce the desired LC alignment angle. Furthermore, this process also processes a single or at most a few wafers (typically, not more than 6 wafers) at a time. Thus, this process is not suitable for high volume batch processing and is not usable for substrates larger than 10-in. Furthermore, this process generally has a high rate of defect due to particle contamination at the surface. In addition alignment layers formed with silicon monoxide have humidity related reliability problems.
Hence there is a need for low cost processes and apparatuses to produce alignment layers in MVA liquid crystal displays. In addition the low cost processes should also allow multiple wafers to be processed simultaneously (i.e. batch processing).